1. Field of the Invention
Aspects of the invention relate to an infrared detection device including an endoscope with an array of infrared radiation detecting elements mounted at the distal end. The infrared array is sensitive at e.g. wavelengths from two to fourteen micrometers. An endoscope is a probing device used to gain access, for example visual access, to an interior cavity of a body through a relatively small hole or access channel. Examples of endoscopes used in medical applications include arthoscopes, laparoscopes, cystoscopes, bronchoscopes, etc. A preferred embodiment of the invention uses a two-dimensional array of microbolometer sensor elements packaged in an integrated vacuum package and co-located with readout electronics on the distal tip of an endoscope. An endoscope with an infrared sensing array can be used to accurately measure relative and absolute temperatures in e.g. medical, industrial, law enforcement, and other applications.
2. Description of Related Art
The electromagnetic spectrum includes ultraviolet (wavelengths from 0.1 to 0.4 micrometers), visible (from 0.4 to about 0.75 micrometers), near-infrared (From 0.75 to 1.2 micrometers), mid-infrared (from 2 to 5 micrometers) and far-infrared (from 8 to 14 micrometers). All materials at temperatures above zero degrees Kelvin emit infrared radiation. Most naturally occurring terrestrial objects have peak infrared emissions in the 8 to 14 micrometer range. Hot objects, such as jet engines, have peak infrared emissions in 3 to 5 micrometer range.
Early IR imaging systems developed in the 1970s and 1980s were unwieldy and did not lend themselves well to many applications. Physically large and technically complex, they required expensive liquid nitrogen or similar cryogenic cooling systems. IR imaging systems have been slow in delivering greater operational flexibility because of the cost, size, and weight of the cryogenic cooling components used in prior generations of high-performance IR sensors, and because of the size and power consumption of the supporting electronics.
In the early 1990s a revolutionary suite of imaging radiation sensors was developed (see U.S. Pat. Nos. RE036615, 6,114,697, 5,554,849, and 5,834,776, all of which are incorporated herein by reference). These sensors were revolutionary because they are mass-producible from materials such as low-cost silicon and they operate well at room temperatures (hence termed xe2x80x9cuncooledxe2x80x9d).
Uncooled IR sensors, such as of the microbolometer type that Honeywell has invented, typically consist of arrays of microscopic bridge-like structures micromachined from silicon. Given the extremely low mass of the microbridge structures (typically on the order of a nanogram), they respond to very low radiation levels. Accurate measurements of microbridge temperature changes are used to quantify incident IR radiation. Common methods for measuring microbridge temperatures include the use of thin-film thermocouples to generate a thermoelectric (TE) signal, or the use of thin-film resistors that undergo resistance changes according to temperature.
The basic operating principle of an uncooled silicon IR detector is as follows. Infrared energy emitted from the target object is focused onto an extremely low mass microstructure. The incident energy is absorbed by the microstructure and causes an increase in the temperature of the bulk of the material. This temperature rise can be exactly correlated to the temperature at the optically corresponding point on the target. Honeywell""s uncooled IR imaging sensors consist of arrays of microscopic (typically 0.05 mm wide and 0.001 mm thick) bridge-like structures xe2x80x9cmicromachinedxe2x80x9d into silicon wafers by photolithographic processes similar to those used to make microprocessors. Calculation of the heating of microbolometers produced by focused IR radiation can be made using the well-known physical laws of radiation, and we find that such microbolometers can measure temperature changes in a remote object with sensitivity well below 0.1 C.
For best sensitivity, microbolometer arrays should operate in an air pressure of 50 mTorr or less in the vicinity of the pixels, to eliminate thermal loss from the pixel to the air. To minimize size and weight and production costs, Honeywell has developed and patented (U.S. Pat. No. 5,895,233, incorporated herein by reference) a process allowing the completed array to be have an infrared-transparent silicon top cap hermetically attached, to form an all-silicon integrated vacuum package (IVP). This technique allows a microbolometer imaging array to have small dimensions. Existing microbolometer packages require a vacuum-sealed package around the outside of the microbolometer, resulting in larger diameters. Arrays are typically close-packed across the wafer, with a very small spacing to allow wafer sawing to separate completed arrays.
Since the sensors are fabricated using silicon photolithography, it is cost effective to fabricate large one-dimensional (1D) and two-dimensional (2D) arrays complete with monolithic silicon readout electronics if required for a particular application. Two-dimensional arrays of IR sensors may be used with an IR-transmitting lens to produce a 2D temperature map of a target, analogous to the way a visible camera produces a two-dimensional image of a target.
Other methods have also been developed to construct arrays of infrared radiation detectors, including the use of pyroelectric detector elements, p-n junction devices, microcantilevers, or photoconductive or photovoltaic bandgap materials.
Recent advances in minimally invasive surgery, for example techniques that utilize heat treatment, have resulted in a need to monitor tissue temperatures with increased accuracy. Heat treatment procedures involve, but are not limited to, the use of lasers, radio frequency (RF) devices, and ultrasonic heating methods that are typically applied using an endoscope. An example of the use of heat treatment is in the destruction of internal cancerous tumors. The challenge is to monitor the temperature of the treatment area while the heat is being applied to avoid overheating surrounding tissue and causing irreparable damage. Current methods for monitoring heat treatments include looking for visible color changes in the tissue with a visible light endoscope and the use of thermocouples on the end of an endoscope. Tissue temperatures may be monitored using infrared detectors.
The use of a visible-light imaging array on the distal end of an endoscope is well established, for example using a silicon solid-state array called a charge coupled device (CCD). Methods and devices are taught for example in U.S. Pat. Nos. 4,971,035, 5,305,736, 5,827,190, 4,918,521, 4,868,644, 5,051,824 and 6,019,719, all of which are incorporated herein by reference. U.S. Pat. No. RE035076, incorporated herein by reference, discloses that an IR filter can be used with a CCD camera on an endoscope to sense in the near-IR range. However, such a system is limited to near IR (wavelengths from 0.75 to 1.2 micrometers). The near-IR image does not have the utility for monitoring temperatures as does mid-IR and far-IR.
The prior art also has taught the construction of endoscopes capable of making IR measurements in the mid-IR and far-IR ranges. One approach is to use a series of germanium lenses (germanium is transparent to IR radiation; glass is not) in a rigid endoscope to relay IR radiation from the distal end to an external IR camera (U.S. Pat. Nos. 5,833,596, 5,711,755, and 5,944,653, all incorporated herein by reference). A second approach is taught by U.S. Pat. No. 5,445,157, incorporated herein by reference, wherein an infrared transmitting fiber of chalcogenide or fluoride glass relays IR radiation from the distal end of a flexible endoscope to an external IR camera.
Conlan et al. (WO 98/32380), incorporated herein by reference, teach a single-point articulating thoracic endoscope where the imaging assembly is a thermal imaging assembly.
The construction of infrared sensitive arrays typically requires that a vacuum surround the bolometer elements. This can be achieved using a metal package containing, for example, a germanium window and glass wire feed-thrus. Another approach, by Higashi et al in U.S. Pat. No. 5,895,233, incorporated herein by reference, teaches a method that brings together two wafers of dies that contain an infrared transparent window or top cap with either an infrared detector or emitter array to produce a low-cost vacuum package.
Aspects of the present invention provide methods for constructing an infrared sensitive endoscope to provide measurements of infrared radiation in the 2 to 14 micrometer wavelength range by mounting an array of infrared radiation detecting elements at the distal end of an endoscope. This device can be used to form a thermal image or temperature map showing the relative or absolute temperatures of an object under observation. An infrared measurement system that yields absolute temperatures is called a radiometric system. Sensor signals and electrical power are transferred from the distal tip for connection to an output device. These signals can then be manipulated and displayed visually, for example in a false-color temperature image, or used as input to a computer program for various applications.
Several different sensor technologies are envisioned for the construction of a suitable infrared radiation detecting array. Additionally, the individual sensors that make up the array can be organized in different patterns and in different numbers to achieve varying observational objectives. Additionally, the plane of the array may be situated at any angle with respect to the axis of the endoscope. The array of infrared radiation detecting elements can be used in combination with a conventional visible endoscope to provide visible and infrared information, for example simultaneous spatially aligned infrared and visible information.
An infrared transparent gas, for example carbon dioxide, flowing down a channel in the endoscope, can vent on to the outer window of the infrared array/optics assembly to remove any condensation or liquids that may collect during use. The infrared radiation detection array may include a temperature stabilization apparatus, for example a thermoelectric plate, to increase the accuracy and dynamic performance of the array. The infrared detectors may also be constructed using an integrated vacuum package. The use of three-dimensional hybrid package techniques for connecting the signals from a IR sensitive array to conditioning electronics and cabling behind the array can be used to achieve the diameter practical for an endoscope.
Aspects of the present invention use an array of infrared radiation detecting elements at the distal end of an endoscope. The array of infrared detecting elements can detect radiation in the 2 to 14 xcexcm wavelength band (mid and far IR, according to one embodiment, as opposed to xe2x80x9cnear IRxe2x80x9d which extends from 0.75 to 2 xcexcm. The near-IR band can be achieved with an IR filter and a silicon CCD (visible light) camera, according to one example.
The array of the infrared detecting elements can be used with or without cooling.
Suitable types of infrared detectors include:
An infrared array made of microbridges, with the temperature of each microbridge individually sensed. These are called microbolometers.
Methods for sensing microbridge temperatures include:
Using thin-film thermocouples on each microbridge to generate a thermoelectric (TE) signal. This is known as a TE sensor.
Using thin-film resistors on each microbridge whose resistance changes with temperature.
Using a selected forward-biased p-n junction, on each microbridge.
An infrared array where the infrared detector element is a pyroelectric element.
An infrared array where the infrared detector element is a thin film diode transistor coated with infrared absorbent material, and mounted on a thermally insulating support such as silicon oxide, foam glass or foam plastics material.
An infrared array where the array elements are a resonant optical cavity.
An infrared array where the array elements are a periodic pattern of photoconductive or photovoltaic, bandgap detector elements spaced apart at a period which is equal to or less than the wavelength of IR radiation.
An infrared array where the array elements are microcantilevers.
Methods for providing a vacuum in which the infrared elements operate include:
Packaging the array elements using an integrated vacuum seal on the surface of the die to achieve a diameter practical for an endoscope.
Packaging the array elements inside vacuum-sealed capsule that uses the optical element and interconnect feed thrus as the points of the vacuum seal.
Methods for constructing the infrared array on a semiconductor die include:
Constructing the array of infrared radiation detecting elements from silicon or GaAs material
Constructing the array of infrared radiation detecting elements co-located with integrated silicon or GaAs circuitry.
The number and organization of infrared detecting elements on the array can be unique to each application; possible configurations include:
Organizing the array as a linear array of one by N infrared detecting elements where N is any whole number including 1.
Organizing the array of infrared detecting elements as a two dimensional array of N by M were N and M are any positive integer including one.
Organizing the array of infrared detecting elements in an irregular pattern to allow it to compensate for the distortions caused by constraints on the optics.
Organizing the array of infrared detecting elements in a circular pattern.
Varying the density of the array elements in any pattern to provide differing levels of radiation sensing detail, for example to provide more detailed temperature mapping at critical points under observation.
The infrared detecting array can have varying orientations with respect to the central axis of the endoscope. Such orientations include:
Mounting the array of infrared detecting elements in a plane perpendicular to the longitudinal axis of the endoscope.
Mounting the array of the infrared detecting elements in a plane at any angle with respect to the longitudinal axis of the endoscope.
Greater resolution can be achieved by having the array of the infrared detecting elements at an angle to the distal end of the endoscope.
Three-dimensional hybrid construction techniques can be used to assemble the distal infrared sensor assembly.
The array of the infrared detecting elements can be used to make radiometric temperature measurements. Techniques to facilitate radiometric measurements include:
Control the temperature of the array of the infrared detecting elements using for example:
Control the temperature of the array of the infrared detecting elements with a thermoelectric device.
Control the temperature of the array of the infrared detecting element with a flow of a gas near the array.
Monitor the temperature around the array of the infrared detecting elements.
Insert an object(s) of known temperature in front of the array of the infrared detecting elements.
The optics used in front of the infrared radiation detection array are IR transmitting materials such as chalcogenide glass, fluoride glass, zinc selenide glass, germanium, or silicon.
The endoscope using the array of the infrared detecting elements at the distal end can be rigid or of a flexible type.
The array of infrared radiation detecting elements at the distal end of the endoscope can be collocated with a separate visible light endoscope technique such as a CCD array.
Blowing an IR transparent gas onto the lens of the IR endoscope will help prevent condensation and keep the lens free from debris that could absorb the IR radiation.
Other features and advantages according to the invention will be apparent from the remainder of this application.